Cambridge AS/A Level Biology

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Defintions of AS/A Level Biology Key Words

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54 Terms

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cell

the basic unit of all living organisms; it is surrounded by a cell surface membrane and contains genetic material (DNA) and cytoplasm containing organelles

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organelle

a functionally and structurally distnct part of a cell, e.g. a ribosome or mitochondrion

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nucleus (plural: nuclei)

a relatively large organelle found in eukaryotic cells, but absent from prokaryotic cells; the nucleus contains the cell’s DNA and therefore controls the activities of the cell; it is surrounded by two membranes which together form the nuclear envelope

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eukaryote

an organism whose cells contain a nucleus and other membrane-bound organelles

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prokaryote

an organism whose cells do not contain a nucleus or any other membrane-bound organelles

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cell surface membrane

a very thin membrane (about 7 nm diameter) surrounding all cells; it is partially permeable and controls the exchange of materials between the cell and its environment

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chromatin

the material of which chromosomes are made, consisting of DNA, proteins, and small amounts of RNA; visible as patches or fibres whithin the nucleus when stained

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chromosome

in the nucleus of the cells of eukaryotes, a structure made of tightly coiled chromatin (DNA, proteins and RNA) visible during cell division; the term ‘circular DNA’ is now also commonly used for the circular strand of DNA present in a prokaryotic cell

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nucleolus

a small structure, one or more of which is found inside the nucleus; the nucleolus is usually visible as densely stained body; its function is to manufacture ribosomes using the information in its own DNA

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protoplasm

all the living material inside a cell (cytoplasm plus nucleus)

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cytoplasm

the contents of a cell, excluding the nucleus

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mitochondrion (plural: mitochondria)

the organelle in eukaryotes in which aerobic respiration takes place

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cell wall

a wall surrounding prokaryote, plant and fungal cells, the wall contains a strengthening material which protects the cell from mechanical damage, supports it and prevents it from busting by osmosis if the cell is surrounded by a solution with a higher water potential

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plasmodesma (plural: plasmodesmata)

a pore-like structure found in plant cell walls; plasmodesmata or neighbouring plant cells line up to form tube-like pores through the cell walls, allowing the controlled passage of materials from one cell to the other; the pores contain ER and are lined with the cell surface membrane

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vacuole

an organelle found in eukaryotic cells; a large, permanent central vacuole is a typical feature of plant cells, where it has a variety of functions, including storage of biochemicals such as salts, sugars and waste products; temporary vacuoles , such as phagocytic vacuoles (also known as phagocytic vesicles), may form in animal cells

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tonoplast

the partially permeable membrane that surrounds plant vacuoles

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chloroplast

an organelle, bounded by an envelope (i.e. two membranes), in which photosynthesis takes place in eukaryotes

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photosynthesis

the production of organic substances from inorganic ones, using energy from light

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grana (singular: granum)

stacks of membranes inside a chloroplast

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magnification

the number of times larger an image of an object is than the real size of the object; magnification equals to the image size divided by the actual(real) size of the object (image size/actual size)

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eyepiece graticule

small scale that is placed in a microscope eyepiece

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stage micrometer

very small, accurately drawn scale of known dimensions, engraved on a microscope slide

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micrograph

a picture taken with the aid of a microscope; a photomicrograph (or light micrograph) is taken using a light microscope; an electron micrograph is taken using an electron microscope

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resolution

the ability to distinguish between two objects very close together; the higher the resolution of an image, the greater the detail that can be seen

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microvilli (singular: microvillus)

small, finger-like extensions of a cell which increase the surface area of the cell for more efficient absorption or secretion

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nuclear envelope

the two membranes, situated close together, that surround the nucleus; the envelope is perforated with nuclear pores

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nuclear pores

pores found in the buclear envelope which control the exchange of materials, e.g. mRNA, betweeen the nucleus and the cutoplasme

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endoplasmic reticulum (ER)

a network of flattened sacs running through the cytoplasm of eukaryotic cells; molecules, particularly proteins, can be transported through the cell inside the sacs separate from the rest of the cytoplasm; ER is continuous with the outer membrane of the nuclear envelope

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ribosome

a tiny organelle found in large numbers in all cells; prokaryotic ribosomes are about 20 nm in diamter while eukaryotic ribosomes are about 25 nm in diamter

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Golgi apparatus (Golgi body, Golgi complex)

an organelle found in eukaryotic ells, the Golgi apparatus consists of a stack of flattened sacs, constantly forming at one end and breaking up in to Golgi vesicles at the other end

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Golgi vesicles

carry their contents to other parts of the cell, often to the cell surface membrane for secretion; the Golgi apparatus chemically modifies the molecules it transports, e.g. sugars may be added to proteins to make glycoproteins

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lysosome

a spherical organelle found in eukaryotic cells; it contains digestive (hydrolytic) enzymes and has a variety of destructive functions, such as removal of old cell organelles

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cristae (singular: crista)

folds of the inner membrane of the mitochondrial envelope on which are found stalked particles of ATP synthase and electron transport chains associated with aerobic respiration

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ATP (adenosine triphosphate)

the molecule that is the universal energy currency in all living cells; the purpose of respiration is to make ATP

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ADP (adenosine diphosphate)

the molecule that is converted to ATP by addition of phosphate (a reaction known as phosphorylation) during cell respiration; the enzyme responsible is ATP synthase; the reaction requires energy

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microtubes

tiny tubes made of a protein called tubulin and found in most eukaryotic cells; microtubules have a large variety of functions, including cell support and determining cell shape; the ‘spindle’ on which chromatids and chromosomes separate during nuclear division is made of microtubules

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centriole

one of two small, cylindrical structures made from microtubules, found just outside the nucleus in animal cells, in a region known as the centrosome; they are also found at the bases of cilia and flagella

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centrosome

the main microtubule organising centre (MTOC) in animal cells

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cilia (singular: cilium)

whip-like structures projecting from the surface of many animal cells and the cells of many unicellular organisms; they beat, causing locomotion or the movement of fluid across the cell surface

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flagella (singular: flagellum)

whip-like structures projecting from the surface of some animal cells and the cells of many unicellular organisms; they beat, causing locomotion or the movement of fluid across the cell surface; they are identical in structure to cilia, but longer

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thylakoid

a flattened, membrane-bound, fluid-filled sac which is the site of the light-dependent reactions of photosynthesis in a chloroplast

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bacteria (singular: bacterium)

a group of single-celled prokaryotic microorganisms; they have a number of characteristics, such as the ability to form spores, which distinguish them from the other group of prokaryotes known as Archaea

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peptidoglycan

a polysaccharide combined with amino acids; it is also known as murein; it makes the bacterial cell wall more rigid

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plasmid

a small circular piece of DNA in a bacterium (not its main chromosome); plasmids often contain genes that provide resistance to antibiotics

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virus

a very small (20-300 nm) infectious particle which can replicate only inside living cells; it consists of a molecule of DNA or RNA (the genome) surrounded by a protein coat; an outer lipid envelope may also be present

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phospholipid

a lipid to which phosphate is added; the molecule is made up of a glycerol molecule, two fatty acids and a phosphate group; a double layer (a bilayer) of phospholipids forms the basic structure of all cell membranes

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macromolecule

a large molecule such as a polysaccharide, protein or nucleic acid

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polymer

a giant molecule made from many similar repeating subunits joined together in a chain; the subunits are much smaller and simpler molecules known as monomers; examples of biological polymers are polysaccharides, proteins and nucleic acids

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momnomer

a relatively simple molecule which is used a basic building block for the synthesis of a polymer; many monomers are joined together by covalent bonds to make the polymer, usually by condensation reactions; common examples of monomers are monosaccharides, amino acids and nucleotides

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condensation reaction

a chemical reaction involving the joining together of two molecules by of a water molecule

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hydrolysis

a chemical reaction in which a chemical bond is broken by the addition of a water molecule; commonly used to break down complex molecules into simpler molecules

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monosaccharide

a molecule consisting of a single sugar unit and with the general formula (CH2O)n

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disaccharide

a sugar molecule conssting of two monosaccharides joined together by a glycosidic bond

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glycosidic bond